As the IC device sizes have been shrunk to nanometer scale, the consumer electronics, like the laptop and mobile devices, have been designed to be much smaller than ever. Without suitable protection devices, the functions of these electronics could be reset or even damaged under ESD events. Currently, all consumer electronics are expected to pass the ESD test requirement of IEC 61000-4-2 standard. TVS is generally designed to bypass the ESD energy, so that the electronic systems can be prevented from ESD damages. The working principle of TVS is shown in FIG. 1. In FIG. 1, the TVS devices 10 are connected in parallel with the protected circuits 12 on the PCB (Printed Circuit Board). These TVS devices 10 would be triggered immediately when the ESD event is occurred. In that way, each TVS device 10 can provide a superiorly low resistance path for discharging the transient ESD current, so that the energy of the ESD transient current can be bypassed by the TVS devices.
FIG. 1B is a cross-sectional view showing a traditional vertical TVS. The traditional vertical TVS included a N+ type substrate, an Epi layer, a P− type doped well and a N type heavily doped region. The P− type doped well is arranged in the Epi layer on the N+ type substrate, and the N type heavily doped region is arranged in the P− type doped well. FIG. 1C is a schematic diagram showing a traditional equivalent circuit with a pair of vertical transistors of FIG. 1B. In the traditional equivalent circuit, one of pair of vertical transistor is oppositely coupled to another of pair of vertical transistor. However, the transitional vertical TVS has some disadvantages as follows: (1) the breakdown voltages (Vceo) are not symmetric because the structures of the traditional vertical TVS are not symmetric; (2) the I-V characteristic are not symmetric for positive-side curve and negative-side curve; (3) the ability of ESD protection under positive and negative ESD stresses will not be the same; (4) the variation in breakdown voltage of the structure of the traditional vertical TVS is large because the out-diffusion effect of heavily doped substrate during the process.
For the reason that the traditional vertical TVS could not provide effectively ESD protection, a need has arisen to propose a novel scheme that may adaptively provide effectively ESD protection and overcome the drawbacks of the prior art.